The disclosure is related to a method for accurate positioning of control devices for instrumented cables.
The disclosure further relates to a system for accurate positioning of control devices for instrumented cables.
The position of instrumented cables in the sea is controlled by control devices which can control the instrumented cables horizontally and vertically based on control signals being sent from a survey vessel via the instrumented cable to each control device, also known as birds. The instrumented cables are towed by a survey vessel and have a control device for each 300 meters along the instrumented cable, which can be up to 5-6 km long. To obtain sufficient data basis to evaluate oil deposits in the ground, usually 10-12 instrumented cables are to be towed in parallel in a spread. Due to varying conditions with regard to currents and salinity in the sea, the position of the instrumented cables can deviate relatively much from the straight line behind the survey vessel which one desire to achieve during the towing. As the quality of the data basis to a large extent are depending on that one know the accurate position of the instrumented cable, it is important to know the position of the instrumented cable with as high accuracy as possible. One way to determine this is by determining the position of the control device as accurately as possible. The position of the instrumented cable is then usually approximated as the straight line between the control devices.
Present known solutions use acoustic triangulation between acoustic transponders arranged in the instrumented cable and work boats or following vessels.
U.S. Pat. Nos. 4,532,617 and 4,669,067 both describe acoustic triangulation methods where following (slave) vessels and acoustic transponders arranged in the instrumented cable are used. The disadvantage with this method is that it results in a high number of transponders and operative personnel, and following vessels which are necessary to perform the operation.
US 2005/0180263 A1 describes a method with GPS receiver on towed following vessel and acoustic means for communication with transponder arranged in the instrumented cable. The disadvantage with this method is that the position deviation caused by currents and wave affection, and that one need a towed GPS unit with transponder for each segment of the instrumented cable to be positioned. A high number of following vessels will make deploying and retrieving of the instrumented cable an extensive, expensive and inexpedient operation.
Another disadvantage with the mentioned solutions is that, as the transponders are arranged in the instrumented cable, they are subjected to leakage in the cable. Further, if an error should arise, the entire cable segment must be exchanged.
Other methods for positioning are by use of compass in the control device and instrumented cable, and the use of accelerometer and velocity meters for estimating the position of the instrumented cable. In addition, floaters or tail buoys are used at the end of the instrumented cable where one can utilize GPS positioning. The position of the tail buoy can then be estimated with high accuracy, but one cannot necessarily use this to estimate the position of the instrumented cable for the distance between the vessel and the tail buoy due to the current conditions in the sea. This method will in best case provide sufficient accuracy for the position of the instrumented cable near the tail buoy.
The mentioned methods have several disadvantages in a practical seismic operation. Accelerometer and compass provides relatively low precision and can only be used for approximate estimation of position.
Acoustic positioning requires extensive use of equipment and manpower in the form of following vessels and has limitations with regard to the range of the acoustic signals.
Another known method is by using a towed surface vessel with GPS positioning. The surface vessel is then connected with the control device by means of towing line and the position of the control device is assumed to be the same as the position of the surface vessel. As the control device is located up to 50 meters under the surface it could however be a considerable deviation in position between the control device and surface vessel due to the current condition and the drag in the line. The surface vessel will in this case only exceptionally be located straight above the control device due to currents and the drag in the line.
Another disadvantage with this method is that it in practice only can be utilized for a low amount of control devices at the time due to each control device must be physically connected with one surface vessel via a line. This increases both the complexity in the operation of deploying and retrieving control devices and introduces noise from the towing which will increase the noise level of the acoustic measurements.
There is accordingly a need for a method and a system for accurate positioning of control devices for instrumented cables solving the weaknesses/disadvantages of prior art, and simplifying the task of accurate positioning of the control devices.